- require karma tests
 - temporarily turn off module transformation on airbnb-preset

I want to do some optimizations on how react-dates is built, to help
reduce the bundle size impact of using this project. The first step I
see is reducing the size of each of the components, which we can achieve
by avoiding building them with webpack.

From what I can tell, webpack was only brought in to support getting
SVGs in these components. We can pretty easily replace this with a Babel
transform that will achieve the same result, but with less complexity in
our build, and a little less overhead in the resulting components.

Before this change:

```
~/react-dates ❯❯❯ du -hs lib/components
420K    lib/components
```

After this change:

```
~/react-dates ❯❯❯ du -hs lib/components
348K    lib/components
```

Since the primary use-case for react-dates is in a browser environment
it seems appropriate to run the test suite in a browser.

I've added configuration to run the test suite in Firefox via Karma
with the necessary adjustments to the Travis config. I've commented
the two places in the Karma config where I'm doing something odd in
order to make things work.